The present invention refers to a safety switch assembly, and in particular to a safety switch assembly of a type having a housing adapted for receiving a key by which a plunger is axially movable for operating an electric switch to interrupt a machine motor in dependence on the position of the plunger, and an electromagnetically actuated locking mechanism which allows a withdrawal of the key from the housing only after energizing the locking mechanism.
Operating regulations require the incoporation of such safety switch assemblies in protective screens which guard the interior of machines from the outside, if there exists a danger that these protective screens are opened during operation. In order to prevent this from occurring, safety switch assemblies are designed such that the protective screens are locked until the running parts of the machine come to a standstill and any danger of injury when coming into contact with these machine parts is eliminated. The protective screen can thus be opened only when removing the key from the housing by which the safety switch assembly is enclosed, with the plunger being guided for axial displacement in order to actuate the electric switch to cut the current supply to the machine motor.
In order to avoid unlocking of the protective screen in situations in which the machine still runs on after shutdown, a fact resulting also in a significant endangerment, the use of solenoid-operated locking units is known (European Patent No. EP 0 553 885, German Patent No. DE 38 10 287), by which the axial movement of the plunger is blocked until a control circuit querying the standstill of the machine signals a release. Only then disengages the locking unit and is the plunger movable so that the key can be withdrawn and the protective screen unlocked.
Conventional safety switch assemblies have the drawback that the arrangement of the locking unit is not freely selectable and thus the safety switch assembly cannot be best suited to changing conditions of the machine.